The present invention relates to the construction of conductors for electrical cable to be used for the transmission of power, particularly at high voltages. More particularly, the invention relates to a gas insulated, high voltage cable having an inner conductor which is comprised of multiple conductor elements.
Generally speaking, the transmission of large quantities of electrical energy requires a large voltage and/or a large cross-section of the conductor or conductors used for that purpose. If the conductor(s) is included in a cable one needs correspondingly strong insulation, whereby basically a choice has to be made between a solid type insulation or a gas insulation. A solid insulation can be constructed from laminated dielectric material being, e.g., impregnated with oil. Alternatively, one may extrude such insulation upon the conductor. As far as the latter case is concerned, regular or cross-linked polyethylene is a commonly used insulating material. A gas insulated cable includes, for example, a conductor which is concentrically or coaxially arranged in a tube, and the space between the tube and the conductor is filled with a suitable gas, e.g. SF.sub.6. In addition, of course, the conductor must be held in the tube by spacers.
The high voltage cables outlined above differ substantially in their constructions, but they all have a common feature, namely, they are heated under load and tend to expand particularly in longitudinal direction. This tendency is, of course, the stronger, the higher the operating temperature. The expansion has to be taken up at some point by the cable, and in this connection, it was found that the mounting, holding or other connecting fittings or devices at the ends of the cable will, in fact, experience that load, possibly even to such an extent that the cable end elements are severely damaged. However, other parts of the cable, particularly those portions which do not run along a straight line, experience the thermal load because the resulting force in the cable has a radial component in the curved cable portion. As stated, gas-insulated cable has a conductor which is centrally positioned in the cable tube, and in isolated places only, e.g., by means of plastic spacers. If length extension of the cable is impeded, these spacer elements may have to take up excessive mechanical loads and are deformed or possibly even damaged.
As a representative example for the state of the art, we refer to U.S. Pat. No. 3,852,511. It should be noted that the corrugation of the outer tube of the cable takes care of the thermal expansion problem of that tube, but does not solve the problem of the thermal expansion of the inner conductor. Earlier patents of general interest are, for example, U.S. Re-Issued Patent No. 20,244 or U.S. Pat. No. 2,067,169. In a paper of general interest by C. T. W. Sutton, Energy International, March 1971, power cables are discussed in general and the thermal expansion problem is mentioned, but the paper does not provide a solution.